January 18, 2013

Arctic Melt Ponds Are Forcing Rapid Sea Ice Loss

Melt ponds, a favorite phenomenon among arctic photographers, are turquoise or dark blue pools of water that appear on ice floes during the Arctic summer.

According to a new report in Geophysical Research Letters, scientists from the Alfred Wegener Institute (AWI) have been observing more and more melt ponds atop one-year ice floes, instead of meter-thick, multi-year ice that used to dominate, even during the warmest months in the Arctic.

“The ice cover of the Arctic Ocean has been undergoing fundamental change for some years,” Marcel Nicolaus, a researcher at the Bremerhaven, Germany institution, explained in a statement. “Thick, multi-year ice is virtually nowhere to be found any more. Instead, more than 50 percent of the ice cover now consists of thin one-year ice on which the melt water is particularly widespread.”

“The decisive aspect here is the smoother surface of this young ice, permitting the melt water to spread over large areas and form a network of many individual melt ponds,” he added.

After studying sunlight penetration underneath the floes, the scientists from AWI found that the ice below a melt pond allows far more light and energy into the ocean instead of reflecting it back into space.

“We knew that an ice floe with a thick and fresh layer of snow reflects between 85 and 90 per cent of sunlight and permits only little light through to the ocean,” Nicolaus said. “In contrast, we could assume that in summer, when the snow on the ice has melted and the sea ice is covered with melt ponds, considerably more light penetrates through the ice.”

To see how the sunlight penetrates the floating Arctic sea ice and how this is affected by melt ponds, the AWI researchers utilized a remote-controlled submersible with radiation sensors and cameras. The underwater vehicle was sent directly under the ice floes, recording how much solar energy penetrated the ice at thousands of individual points.

According to Nicolaus–younger, thinner, melt pond-covered ice permits about three times as much light and energy than older, thicker ice. Floes with melt ponds also absorb 50 per cent more solar radiation, the scientist said.

“This conversely means that this thin ice covered by melt ponds reflects considerably fewer sun rays than the thick ice,” Nicolaus explained. “Its reflection rate is just 37 percent. The young ice also absorbs more solar energy, which causes more melt. The ice melts from inside out to a certain extent.”

The climatologist added that the emergence of younger floes that are covered with melt ponds will only contribute to the forces driving climate change.

“We assume that in future climate change will permit more sunlight to reach the Arctic Ocean — and particularly also that part of the ocean which is still covered by sea ice in summer. The reason: the greater the share of one-year ice in the sea ice cover, the more melt ponds will form and the larger they will be,” he said.

“This will also lead to a decreasing surface albedo (reflectivity) and transmission into the ice and ocean will increase. The sea ice will become more porous, more sunlight will penetrate the ice floes, and more heat will be absorbed by the ice. This is a development which will further accelerate the melting of the entire sea ice area,” Nicolaus concluded.